<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
     div{
      width: 100px;
      height: 100px;
      border:1px solid red;
     }
     #d1{
         /*绝对定位,元素脱离文档流*/
       position: absolute;
       right: 20px;
       top: 0;
     }
     #big{
         width:200px;
         height: 200px;
         background-color: green;
         margin: 100px 0 0 100px;

     }

     #big>div{
         width: 100px;
         height: 100px;
         background-color: blue;
         margin: 50px 0 0 50px;
         /*给元素设定相对定位后,绝对定位的元素会以此元素作为参照物*/
         position: relative;
     }
     #big>div>div{
         width: 50px;
         height: 50px;
         background-color: red;
         position: absolute;
         left: 0;
         top:0;

     }
  </style>
</head>
<body>
<div id="big">
    <div>
        <div></div>
    </div>
</div>
<div id="d1">div1</div>
<div>div2</div>
<div>div3</div>
</body>
</html>